Pressure-sensitive adhesives exhibiting an improved shear strength at elevated temperatures

ABSTRACT

The present invention relates to a process for preparing an adhesive bond by i) applying an aqueous dispersion or solution containing a) at least one isocyanate-reactive polymer and b) at least one solid surface-deactivated polyisocyanate to a ii) substrate, iii) drying the aqueous dispersion or solution at a temperature from 20 to 95° C. to form an adhesive layer and iv) treating the adhesive layer with heat at a temperature greater than the temperature used to dry the aqueous dispersion or solution. The present invention also relates to a process for preparing a bond by combining at least one substrate with the adhesive bond of the present invention. 
     Adhesive bonds of the present invention have a shear strength at least 10% greater than the shear strength of adhesive bonds containing a substrate having at least one adhesive layer based on an aqueous dispersion or solution composed of a) and b) dried at a temperature from 20 to 95° C.

FIELD OF THE INVENTION

The invention relates to a process for the preparation ofpressure-sensitive adhesives having improved shear strength at elevatedtemperatures, produced from aqueous dispersions containingisocyanate-reactive polymers and solid surface-deactivated isocyanates.

BACKGROUND OF THE INVENTION

Industry and trade are nowadays seeking high-performancepressure-sensitive adhesives which are used in the construction sectorfor structural bonding in the form of single-sided and double-sidedadhesive tapes as well as in the form of transfer adhesives. Suchhigh-performance pressure-sensitive adhesives are distinguished byimproved creep strength against static mechanical stresses even attemperatures above 100° C.

State of the art are high-performance pressure-sensitive adhesiveswhich, after application to the carrier material, are crosslinked in thesolvent-free state by irradiation in the ultraviolet region. Thedisadvantage of such systems is that the irradiation of thepressure-sensitive adhesive layer takes place on one side. The intensityof the UV light is greatest at the surface of the pressure-sensitiveadhesive layer, that is to say at the surface which later constitutesthe adhesive surface. Accordingly, the crosslinking density is alsogreatest at that surface. The instant adhesion and the surface tackinesssuffer as a result, which can lead to immediate or subsequent adhesivefailure of the adhesive bond.

That is the case especially when pressure-sensitive adhesive layershaving a layer thickness greater than 80 μm or 100 μm are irradiatedwith UV light. High-performance pressure-sensitive adhesives havinglayers greater than 100 μm are used especially where the substratesurface exhibits increased roughness.

According to EP 904 853, improvements in such systems can be achieved byselective UV irradiation, for example through a perforated mask ortemplate. There result highly crosslinked regions having a highmechanical bearing strength and reduced tackiness, together with regionsof lower crosslinking density but having good surface tackiness.

The outlay in terms of apparatus and the costs for UV irradiation ingeneral, and especially for UV radiation using a perforated mask, can beconsiderable. A further disadvantage is also that the UV irradiation canonly be carried out on the high-performance pressure-sensitive adhesivein tape form. An alteration of the mechanical properties at the bondingis not normally possible for various reasons.

A further solution for producing high-performance pressure-sensitiveadhesives is thermal after-crosslinking of the adhesive layer. Thereresult pressure-sensitive adhesives which exhibit good instant adhesionand acceptable creep resistance before thermal activation and have highstrength and static bearing strength after activation. The long exposuretimes of from 10 to 30 minutes at high activation temperatures in therange from 120 to 150° C. are a disadvantage of systems used hitherto.Examples are heat-activatable HAF films (based on epoxy or phenolicresin, copolyamide with epoxy crosslinker, phenolnitrile rubber) fromTesa Industries, Beiersdorf AG (D-Hamburg) or the heat-curing adhesivetape SBT 9245 from 3M (USA-St. Paul Minn.), described in G. Bennet etal.; Strukturelles Haftklebeband—Eine Innovation in der Klebetechnik;Conference Volume 10. Int. Symposium Swissbonding, CH-Rapperswil, 1996,p. 197-205.

The field of application of systems used hitherto is limited by the longexposure times. Many substrates become damaged if they are exposed totemperatures in the range from 120 to 150° C. for from 10 to 30 minutes.For that reason, systems having lower activation temperatures and/orexposure times are desirable.

An overview of pressure-sensitive adhesives, and especially ofstructural pressure-sensitive adhesives that are after-crosslinkable byUV irradiation or thermally, is given by P. L. Geiss, in“Verarbeitungskonzepte und Belastungskriterien für Haftklebstoffe”Hinterwaldner Verlag, Munich, 1998.

In the as yet unpublished Application PCT/CH99/00577 of the sameApplicant, a reactive system for the preparation of spontaneouslycrosslinking pressure-sensitive adhesives is described. That systemconsists of an aqueous dispersion containing at least oneisocyanate-reactive polymer and at least one surface-deactivated solidisocyanate. Features of the said dispersions are

-   -   glass transition temperature Tg of the isocyanate-reactive        polymer less than or equal to −5° C.    -   storage modulus G′ of the isocyanate-reactive polymer less than        or equal to 10⁷ Pa, measured at 10 Hz and 10° C.    -   minimum film-forming temperature MFT less than or equal to +5°        C.    -   mean particle diameter of the solid isocyanates less than or        equal to 10 μm.

The object of the invention according to PCT/CH99/00577 is thepreparation of aqueous dispersions or solutions containing solid,surface-deactivated polyisocyanates and isocyanate-reactive polymers,which are stable to storage in that form, that is to say in the form ofaqueous dispersions or solutions, but in which, when applied in the formof a layer, partial crosslinking is initiated after removal of asubstantial portion of the water.

SUMMARY OF THE INVENTION

Within the scope of further studies it has now been found, surprisingly,that the viscoelastic and mechanical pressure-sensitive adhesiveproperties of the resulting pressure-sensitive adhesive layer can beinfluenced to a considerable degree in a targeted and reproduciblemanner by heat treatment (b) of the pressure-sensitive adhesive layer.

A process is indicated hereinbelow for the preparation of apressure-sensitive adhesive layer and for increasing its shear strength.The process according to the invention comprises the steps

-   i) provision of an aqueous dispersion or solution containing at    least one isocyanate-reactive polymer and a solid    surface-deactivated polyisocyanate-   ii) application of the aqueous dispersion or solution to a substrate-   iii) production of a pressure-sensitive adhesive layer by removal of    the water, the applied layer being dried preferably at a drying    temperature (a) from +20° C. to +95° C. (measured in the layer),    especially from +50° C. to +95° C., characterised in that in step-   iv) heat treatment (b) of the pressure-sensitive adhesive layer is    carried out, wherein the layer is brought continuously, stepwise or    directly to a temperature exceeding the drying temperature (a),    which temperature exceeds the drying temperature (a) preferably by    at least +40° C., and whereby the shear strength of the resulting    adhesive bond is increased as a result of the heat treatment (b) by    at least 10%, compared with the shear strength of the    pressure-sensitive adhesive layer without heat treatment (b) and in    that the isocyanate-reactive polymer and the solid    surface-deactivated isocyanate are so selected that the    pressure-sensitive adhesive layer produced in step iii) exhibits a    loop tack of especially at least 2 N/inch.

DETAILED DESCRIPTION OF THE INVENTION

In a first step, the water is removed by drying (a) at dryingtemperatures in the range from 20° C. to 95° C., especially from 50° C.to 95° C., from the layer applied to a substrate by an aqueousdispersion or solution. There results a pressure-sensitive adhesivelayer, preferably having a loop tack of at least 2 N/inch. The loop tackcan be determined according to the FTM-9 method of FINAT (FederationInternationale des Fabricants et Transformateurs d'Adhesives etThermocollants sur Papiers et autres Supports).

Within the scope of the invention, the pressure-sensitive adhesive layeris to be understood as being the substantially water-free form of thedispersion, especially having a loop tack of at least 2 N/inch. The termlayer is defined in DIN EN 971-1: 1996-09.

According to the invention, the water is removed from the dispersionuntil a layer forms.

Suitable substrates for application of a layer of the aqueous dispersionor solution are various materials such as, for example, tapes or films.The aqueous dispersion or solution may be applied to one side or to bothsides. In the said manner it is possible to prepare especially single-or double-sided pressure-sensitive adhesive tapes or transfer tapeshaving pressure-sensitive adhesive layers prepared in accordance withthe invention.

The evaporation of the water and hence the formation of apressure-sensitive adhesive layer can be carried out either at roomtemperature or at elevated temperatures up to a maximum of 95° C.(measured in the layer) and can be accelerated, for example, by streamsof hot air or by streams of air of relatively low humidity. Dryingtemperatures in the range from 50 to 95° C. (measured in the layer) areparticularly preferred.

Air temperatures from 110 to 125° C. are customary in industry on entryinto the drying tunnel, which temperatures are reduced to from 90 to 95°C. towards the end of the drying line. The progression of thetemperature and the exposure time in the drying tunnel can be soselected that the applied layer is not heated to temperatures above 95°C. Evaporation of the water can also be accelerated by simultaneousirradiation with infra-red light or with microwaves. The drying time isdependent substantially on the layer thickness. Drying times of, forexample, 15 minutes at 20° C., 15 minutes at 50° C. or 5 seconds at 95°C. are thus conceivable, depending on the layer thickness.

In a second step, the shear strength of the pressure-sensitive adhesivelayers prepared according to the invention can be influenced in atargeted and reproducible manner by heat treatment (b). The heattreatment (b) is carried out according to the invention at a temperatureexceeding the drying temperature (a). A maximum temperature for the heattreatment (b) that exceeds the drying temperature (a) by at least 40° C.is particularly preferred. The shear strength of the resulting adhesivebond exceeds the shear strength of the pressure-sensitive adhesive layerbefore heat treatment by at least 10%.

The shear strength can be determined by the FTM-8 method of FINAT(Federation Internationale des Fabricants et Transformateurs d'Adhesiveset Thermocollants sur Papiers et autres Supports).

Adhesive bond within the scope of the invention denotes substrateshaving at least one pressure-sensitive adhesive layer prepared accordingto the invention after heat treatment (b) has been carried out. Theyinclude especially single- or double-sided coated adhesive tapes,transfer tapes or bonds of one or more identical or differentsubstrates.

The heat treatment (b) of the pressure-sensitive adhesive layer can becarried out immediately after the removal of water by drying (a) or at alater time. In the heat treatment, the pressure-sensitive adhesivelayers prepared according to the invention are heated continuously,stepwise or directly, in such a manner that the shear strength of theheat-treated pressure-sensitive adhesive layer, or of the resultingadhesive bond, is increased by at least 10% as compared with the valueprior to heat treatment (b).

Continuous heating within the scope of the invention is to be understoodas meaning that heating is carried out with a constant increase intemperature (temperature gradient), for example 2° C./minute or 5°C./minute. In the case of stepwise heating within the scope of theinvention, heating is carried out for different periods of time atdifferent constant temperatures and/or with different temperaturegradients. In direct heating, the pressure-sensitive adhesive is exposedin one step to a constant temperature that exceeds the dryingtemperature (a), especially to a temperature that exceeds the dryingtemperature (a) by at least 40° C.

The duration of the heat treatment (b) is dependent substantially on thereactivity of the isocyanate-reactive groups of the functional polymercontained in the aqueous dispersion. For example, the reactivity betweenprimary hydroxyl and carboxyl groups under identical conditions differsapproximately by a factor of up to 100. The second reaction partner, thesolid isocyanates, has reactivities of the isocyanate groups that canlikewise differ up to a factor of 100. Catalysts may likewise acceleratethe reaction.

For those reasons, the time required for the heat treatment (b) must bedetermined by means of experiments. The duration of the heat treatment(b) ranges from a minimum of 5 seconds at 135° C. (or highertemperatures) to a maximum of 30 minutes at 90° C. (measured in thelayer). Particular preference is given to a maximum temperature in theheat treatment (b) that exceeds the drying temperature (a) for removalof the water by at least 40° C.

The heat treatment (b) is ideally carried out in the range from 60° C.to 200° C., preferably from 90° C. to 180° C., the shear strength of theresulting adhesive bond being increased by at least 10% as compared withthe shear strength of the pressure-sensitive adhesive layer prior to theheat treatment (b).

As a result of the heat treatment (b), the surface tackiness of thepressure-sensitive adhesive layers, for example measured as loop tack,should ideally not be less than 50% of the value prior to the heattreatment (b).

The heat required for the heat treatment (b) may be contact heat,convection heat, radiation heat in the visible or in the infra-redregion. It is also possible to heat substrates by means of inductionheat or by means of microwaves and thus transmit the heat to theadhesive tape and the adhesive layer. The pressure-sensitive adhesivelayer may also be filled with inorganic, metallic or organic fillerswhich respond to the mentioned types of radiation.

The heat treatment (b) leads (in contrast to crosslinking by means of UVirradiation, especially using a perforated mask) to a macroscopicallyhomogeneous adhesive layer having the same properties in all layers andthroughout the entire cross-section.

As a result of the particular selection of the components in the aqueousdispersion or solution, pressure-sensitive adhesive layers are producedthat can be thermally after-crosslinked at lower temperatures,especially at temperatures below 100° C., and/or with shorter exposuretimes, for example 5 seconds at 135° C. (or higher temperatures).

The aqueous dispersions or solutions must fulfil substantially thefollowing requirements:

-   (a) the minimum film-forming temperature (MFT) according to DIN    53787:1974-02 is less than or equal to +5° C.;-   (b) the glass transition temperature (Tg) of the isocyanate-reactive    polymer must be less than or equal to −5° C.;-   (c) the storage modulus G′ of the isocyanate-reactive polymer must    be less than or equal to 10⁷ Pa, measured at 10° C. and 10 Hz.

The viscoelastic parameters (glass transition temperature and storagemodulus) of the films resulting from the dispersion are determined byDTMA (dynamic thermomechanical analysis) according to ISO 6721-1.Caloric measurements (DSC differential scanning calorimetry) canlikewise be used. Standards for suitable methods are laid down in DIN53765: 1994-03 and ISO 11357-2: 1996. A description of the dynamicthermomechanical properties and their determination is additionallycontained in the Encyclopedia of Polymer Science and Engineering, Vol.5; H. F. Mark ed.; New York 1986, p. 299 ff.

The isocyanate-reactive polymers can be prepared by copolymerisation ofolefinically unsaturated monomers in solution, emulsion or suspension.They contain polymerised therein from 0.1 to 15%, preferably from 1 to8%, monomers having isocyanate-reactive groups, such as hydroxyl, amino,carboxyl or acid amide and methylol acid amide groups.

Preference is given to hydroxy-, carboxy- and amino-functionaldispersion copolymers of acrylic acid esters and methacrylic acidesters, acrylonitrile, vinyl acetate, diesters of maleic and fumaricacid, vinyl ethers, copolymers of ethylene with vinyl acetate,functional copolymers of styrene, butadiene, isoprene,α-chlorobutadiene. The molecular weights are generally from 100,000 to800,000 Da, but may also be higher than 800,000 Da, especially whencrosslinking monomers are used concomitantly.

There are also used water-soluble and water-dispersible polyurethane orpolyurea dispersions, which are formed by the reaction of amorphous orsemi-crystalline reactive polyesters, sulfopolyesters,polycaprolactones, polycarbonates and polyethers, which carry sulfoxyl,carboxyl, hydroxyl as well as primary or secondary amino groups, withaliphatic or aromatic polyisocyanates.

The solubility parameter δ of the anhydrous functional polymer shouldadvantageously be in the range from 8.25 to 13.5 (cal/cm³)^(1/2). Forthe definition of the solubility parameter see Römpp Lexikon, Lacke undDruckfarben, Ed. U. Zorll, Stuttgart 1998, p. 361 ff.

The concentration of the isocyanate-reactive polymers in water isapproximately from 20 to 80 wt. %, preferably from 30 to 70 wt. %, ofthe total weight of the dispersion or solution.

The pH value of the dispersion or solution is in the range from 6 to 10,preferably from 7 to 9. If necessary, the pH value can be brought intothe desired range by addition of inorganic or organic bases or acids.

In the context of the present invention, the term dispersion is also toinclude emulsions, suspensions and polymer solutions.

Suitable solid polyisocyanates are any water-insoluble di- orpoly-isocyanates or mixtures thereof, provided they have a melting pointabove +38° C. They may be aliphatic, cycloaliphatic, heterocyclic oraromatic polyisocyanates. Examples which may be mentioned are:diphenylmethane 4,4′-diisocyanate (4,4′-MDI), dimeric 2,4′- and4,4′-MDI, naphthalene 1,5-diisocyanate (NDI), 3,3′-dimethyl-biphenyl4,4′-diisocyanate (TODI), dimeric 1-methyl-2,4-phenylene diisocyanate(TDI-U), 3,3′-diisocyanato-4,4′-dimethyl-N,N′-diphenylurea (TDI-H),addition products of 2 mol of 4,4′-MDI with 1 mol of diethylene glycol,addition products of 2 mol of 1-methyl-2,4-phenylene diisocyanate with 1mol of 1,2-ethanediol or 1,4-butanediol, addition products of 2 mol ofhexamethylene 1,6-diisocyanate with 1 mol of 1,2-ethanediol, theisocyanurate of isophorone diisocyanate (IPDI-T).

The solid polyisocyanates should be in the form of powders having a meanparticle diameter of less than or equal to 10 μm (weight average).Weight average within the scope of the invention is to be understood asmeaning that, of 100 g of solid polyisocyanates, on average 50 g has aparticle diameter less than or equal to 10 μm.

The solid polyisocyanates are generally obtained in the synthesis in theform of powders having the required particle diameter of 10 μm or less,otherwise they must be brought to the particle range according to theinvention of less than or equal to 10 μm (before the deactivationreaction) by grinding, screening or sieving processes. Alternatively,the powdered polyisocyanates can be brought to a mean particle diameterof equal to or less than 10 μm by wet grinding and fine dispersion afterthe surface deactivation. Grinding, grading and measuring processes arestate of the art.

The surface-stabilising reaction can be carried out in various ways:

-   -   By dispersion of the powdered isocyanate in a solution of the        deactivating agent.    -   By introduction of a melt of a low-melting polyisocyanate into a        solution of the deactivating agent in a non-solvent liquid        dispersing agent.    -   By addition of the deactivating agent or of a solution thereof        to the dispersion of the solid finely divided isocyanates.

The solid polyisocyanates are preferably deactivated by the action ofprimary and secondary aliphatic amines, di- or poly-amines, hydrazinederivatives, amidines, guanidines. Ethylenediamine,1,3-propylenediamine, diethylenetriamine, triethylene-tetramine,bis-hexamethylene-triamine, 2,5-dimethylpiperazine,3,3′-dimethyl-4,4′-diamino-dicyclohexylmethane, methylnonane-diamine,isophoronediamine, 4,4′-diamino-diamino-dicyclohexylmethane, diamino-and triamino-polypropylene ether (Jeffamine), polyamidoamines andmixtures of mono-, di- and poly-amines have proved expedient.

The concentration of the deactivating agent should be from 0.1 to 20,preferably from 0.5 to 8 equivalent percent, based on the totalisocyanate groups present in the solid isocyanate.

The equivalent ratio of the isocyanate groups of the surface-deactivatedpolyisocyanates and the isocyanate-reactive groups of the polymersshould be in the range from 0.1 to 1.5. That corresponds generally to aconcentration of the surface-deactivated solid isocyanate of from 0.1 to10 parts by weight, preferably from 0.1 to 8 parts per 100 parts ofsolid isocyanate-reactive polymer.

The crosslinking density is given by the concentration of the solidisocyanate and by the concentration of the isocyanate-reactive groups,which are generally present in excess. The crosslinking density Mcshould not fall below a value of Mc=3000 g/mol in the pressure-sensitiveadhesives according to the invention. Mc is understood as being the meanmolecular weight of the elastically acting chain length between thecrosslinking sites. The degree of crosslinking can be demonstrated bythe increase in temperature resistance under heat, by the absence ofthermoplasticity at elevated temperatures, by the peel or shear strengthunder static load, change in surface tackiness, as well as by higherwater resistance (in comparison with the uncrosslinked polymer).Crosslinking can also be demonstrated using analytical methods such asdynamic thermomechanical analysis (DTMA), gel or solubilitydetermination.

The aqueous dispersion or solution optionally contains additionalcatalysts for the reaction of the isocyanate groups with the functionalgroups of the polymer.

Catalysts for the reaction of the isocyanate groups with the functionalgroups of the polymer are organic tin, iron, lead, cobalt, bismuth,antimony, zinc compounds or mixtures thereof. Preference is given tothose catalysts which are hydrolytically stable in aqueous solution ordispersion. Alkyl mercaptide compounds of alkyltin are particularlysuitable owing to their higher hydrolytic stability. Tertiary aminessuch as dimethylbenzylamine, diazabicyclooctane, as well as non-volatilepolyurethane foam catalysts based on tertiary amines, such as, forexample, methyldiethanolamine, can be used for specific purposes or incombination with metal catalysts.

The concentration of the catalysts is in the range from 0.001 to 1%,preferably from 0.01% to 0.1%, based on the solid reactive system.

The aqueous dispersions or solutions also optionally contain tackifyingresins or low molecular weight, isocyanate-reactive compounds.

The addition of polar solvents, such as N-methylpyrrolidone,N-methylacetamide, dimethylformamide, propylene carbonate, dioxan,glycol monomethyl ether acetate, is also conceivable. However, suchpolar solvents may only be used if no damage to the deactivating layeron the solid isocyanates, the polyurea layer, takes place in the aqueousdispersion as a result.

The term plasticiser is to be understood as meaning liquid or solid,organic substances having low vapour pressure. They can interactphysically with highly polymeric substances, with the formation of ahomogeneous system, without a chemical reaction, preferably by theirdissolving or swelling capacity, but sometimes even without such acapacity.

Low molecular weight polymers or resins, especially adhesion-promotingresins, may additionally be added to the aqueous dispersion or solution.Such polymers or resins have a molecular weight equal to or less than5000 Da (weight average). The resins may be natural or synthetic, liquidor solid, they may be added in dispersed or emulsified form in water.They may be added in concentrations up to 100 solid parts per 100 partsof solid polymer.

As low molecular weight isocyanate-reactive compounds there may be usedwater-soluble or emulsifiable low molecular weight liquid polyols or/andpolyfunctional amines. Examples are butanediol, trimethylolpropane,ethoxylated bisphenol A, methyldiethanolamine, triethanolamine.

In addition, further additives, especially inert additives, may be addedto the aqueous dispersion or solution.

The expression inert additives is to be understood as meaning anyadditives or substances that do not have an effect either on the storagestability or on the crosslinking. The said group includes, for example,wetting agents, surface-active substances, auxiliary substances forcontrolling the flow behaviour or rheology, adhesion-promotingsubstances, organofunctional silanes, protective colloids, organic orinorganic thickeners, anti-foams, biocides, light stabilisers, ageingstabilisers, anti-corrosive agents, fillers, pigments, colourants.

The preparation of the dispersions takes place in known dispersingdevices, such as dissolvers, stirrer mills, bead mills, rotor-statormills, planetary mixing devices. It must be ensured that thesurface-stabilised, finely divided polyisocyanates are not exposed tohigh shear forces, so that the urea layer on the surface is notdestroyed. The mixing temperature of the reactive dispersions is in therange from +15 to +50° C., preferably below 35° C., depending on thenature of the solid polyisocyanates used.

It has proved expedient first to prepare concentrated liquid stockmixtures with the solid surface-stabilised polyisocyanates, whichmixtures are added to the aqueous polymer dispersion or solution onlyonce the components have been incorporated. The preparation of the stockmixtures takes place at temperatures from +5° C. to +30° C.

The pressure-sensitive adhesive layers and adhesive bonds producedaccording to the invention can be used in the form of single-sided anddouble-sided adhesive tapes as well as a carrier-free transfer adhesive.

As carriers for single-sided or double-sided pressure-sensitive adhesivetapes there may be used papers, cardboard, metal films, woven materialsof organic and inorganic fibres, nonwovens, foams of acrylic,polyurethane, polyolefin materials. The carrier-free transfer adhesivescan be applied to adhesive-repellent films, which have usually beensilicone-treated on the surface.

Depending on the application, adhesive layers from 5 to 500 μm,preferably from 10 to 250 μm (measured dry) are applied.

The thermally after-treated pressure-sensitive adhesive layers are usedin high-performance adhesives that are creep-resistant at elevatedtemperatures, in adhesive bonds in assembly and manufacturing processes,in statically loadable adhesive tape configurations in the fasteningssector. They can also be used for fastening bondings having highertemperature resistance and shear strength.

The heat treatment (b) can be carried out not only on the single- ordouble-sided pressure-sensitive adhesive tape or on the transfer tape,but also at the bonding of one or two identical or different substrateswith the above-mentioned tapes, sections or die cut-outs. For example,an assembly bond with a tape having high aggressive tackiness andmoderate shear strength can be converted by heat treatment (b) into astructural bond having high shear strength, temperature resistance andmoderate surface tackiness.

The pressure-sensitive adhesive layers and adhesive bonds producedaccording to the invention can be used for a wide variety of otherapplications and are not limited to those mentioned above. The presentinvention is further illustrated by the following Examples.

EXAMPLES

Methods and Materials Used

Pressure-Sensitive Adhesive Aqueous Polymer Dispersions

AC 7506 Copolymer of butyl acrylate, 2-ethylhexyl acrylate and acrylicacid, carboxy-functional. Approx. 65% solid constituents. Molecularweight>500,000 Da, MFT<5° C., Tg −30° C., storage modulus G′=10⁶ Pa at10 Hz and 10° C.

AC 7522 Copolymer of 2-ethylhexyl acrylate, 2-hydroxyethyl(meth)acrylate, a small amount of acrylic acid and crosslinkingdiacrylate, predominantly hydroxy-functional. Approx. 68% solidconstituents. Molecular weight>500,000 Da, MFT<5° C., Tg −43° C.,storage modulus G′=7.5×10⁵ Pa at 10 Hz and 10° C.

Manufacturer Alberdingk Boley GmbH, D-Krefeld

Processing of the Dispersions

-   -   1) Coating weight approx. 30 g/m² (measured dry) on polyester        film substrate    -   2) Process (a): dried in a stream of air at 60° C. for 10        minutes    -   3) Process (b): heat treatment at 100° C. for 15 minutes    -   4) Tested after 24 hours after drying (a) or heat treatment (b)

Testing of the Pressure-Sensitive Adhesives and Bonds

Testing was carried out in accordance with the test methods of FINAT(Federation Internationale des Fabricants et Transformateurs d'Adhesiveset Thermocollants sur Papiers et autre Supports), with the exception ofthe SAFT value (shear adhesion failure temperature) according to Fordand the tensile-shear strength according to DIN 53503.

FTM 1 Adhesive force (180° peel resistance), with indication of the typeof failure (cohesive, with fracture in the adhesive; adhesive, adhesiveseparates from the substrate) FTM 8 Shear strength or shear resistanceunder static load; indication: minutes to failure of the site ofadhesion FTM 9 Surface tackiness (quick stick and loop method) SAFT Fordmethod. Temperature increase in each case 5° C. per 10 minutes, understatic load with 0.9 N/cm². Indication: temperature ° C on failure ofthe site of adhesion

Examples 1-3

Preparation of Aqueous Dispersions (Stock Solutions) ofSurface-Deactivated Isocyanates with Approx. 33% Solid

Constituents used (1) (5) Water deionised (2) Kelzan S hydrocolloid,dispersion stabiliser (3) Tween 85 polyoxyethylene sorbitan trioleate(4) Jeffamine triamine, NH equivalent weight T-403 146 g/eq. (6) IPDI-TIsophorone diisocyanate trimer (Metalink IT, Acima AG, CH-Buchs) meanparticle size approx. 1.8 μm (7) TDI-H 2,4-TDI-urea (Metalink H, AcimaAG, CH-Buchs) mean particle size approx. 4.5 μm (8) TDI-U2,4-TDI-uretdione (Metalink U, Acima AG, CH-Buchs) mean particle sizeapprox. 4.5 μm Example 1 Example 2 Example 3 Stock solution (g) withIPDI-T with TDI-H with TDI-U (1) water, deionised 94.5 94.5 94.5 (2)Kelzan S 0.5 0.5 0.5 (3) Tween 85 1.0 1.0 1.0 (4) Jeffamine T-403 1.551.55 1.55 (0.01 eq.)* (5) dissolved in water 5.0 5.0 5.0 (6) IPDT-T52.00 (0.217 eq.)* (7) TDI-H 52.00 (0.305 eq.)* (8) TDI-U 52.00 (0.288eq.)* (9) Total 154.55 154.55 154.55 *from 3.3 to 4.6 equivalent % ofthe isocyanate groups deactivated

The stock solutions were stirred with the dissolver into the aqueouspressure-sensitive adhesive polymer dispersions at 1200 rpm.

Examples 4-8

Dispersion of deactivated IPDI-T, dispersed in AC 7506 (copolymer ofbutyl acrylate, 2-ethylhexyl acrylate and acrylic acid,carboxy-functional)

Example 4 5 6 7 8 IPDI-T; phr 0 0.5 0.5 1.5 1.5 process/ (a)/60 (a)/60(a)/60 (a)/60 (a)/60 temperature ° C. (b)/100 (b)/100 Loop tack 7.9 7.76.4 7.1 4.9 FTM-9; N/in. Quick stick 27.6 26.1 19.7 24.0 17.4 FTM-9;N/in. FTM-8; shear 8 12 31 13 63 strength at room temperature, 10 N/cm²load, minutes to failure FTM-8; shear 14 17 >5000 20 >5000 strength at70° C., 2 N/cm² load, minutes to failure SAFT (Ford TM)88 >150 >150 >150 >150 Δ5° C./10 min- utes Failure temp. ° C.Tensile-shear 48.9 51.2 53.5 51.7 54.8 strength, N/cm², DIN 53503

Examples 5 to 8 clearly show the effect of the addition of small amountsof surface-deactivated micronised isocyanates (cf. Example 4) and theenormous increase in shear strength or shear resistance on heattreatment at 100° C., as compared with adhesive layers which were driedat 60° C. (Examples 5 to 8).

The adhesion values (loop tack and quick stick) are only moderatelyimpaired by the heat treatment (b). The SAFT values in Examples 5 and 7are already above the industrially required values of +150° C. withoutfurther heat treatment. The tensile-shear values are scarcely affectedby the addition of the surface-deactivated micronised isocyanates.

Examples 9-12

Dispersion of deactivated TDI-U, dispersed in AC 7522, copolymer of2-ethylhexyl acrylate, 2-hydroxyethyl (meth)acrylate, a small amount ofacrylic acid; predominantly hydroxy-functional.

Example 9 10 11 12 TDI-U; phr 0.168 0.168 0.672 0.672 process/ (a)/60(a)/60 (a)/60 (a)/60 temperature ° C. (b)/100 (b)/100 Loop tack 9.8 7.19.1 6.3 FTM-9; N/in. FTM-8; shear strength 39 995 48 >5000 at roomtemperature, 10 N/cm² load, minutes to failure FTM-8; shear strength160 >5000 >5000 >5000 at +100° C., 2 N/cm² load, minutes to failureFTM-1; adhesive force 22.0 18.1 18.7 16.3 (180° peel resistance) N/in.Type of failure 50% c 25% c (cohesive = c; 75% a 100% a 100% a adhesive= a)

Examples 11 to 13 show that TDI-U as the aromatic isocyanate is morereactive than IPDI-T, in addition it has a lower equivalent weight. Witha smaller amount used it leads to greater crosslinking and hence tohigher shear strength at room temperature and at higher temperatures.The fact that the polymer is hydroxy-functional and hence more reactivethan the carboxy-functional AC 7506 also makes a further contribution.

By means of the choice and concentration of the solid isocyanate and ofthe functional groups of the polymer, it is possible to control thecohesive properties to complete adhesive separation from the substratein the peel test.

Examples 13 to 16

Dispersion of deactivated TDI-H, dispersed in AC 7522, copolymer of2-ethylhexyl acrylate, 2-hydroxyethyl (meth)acrylate, a small amount ofacrylic acid; predominantly hydroxy-functional.

Example 13 14 15 16 TDI-H; phr 0.168 0.168 0.672 0.672 process/ (a)/60(a)/60 (a)/60 (a)/60 temperature ° C. (b)/100 (b)/100 Loop tack, FTM-9;N/in. 10.1 7.8 10.0 6.1 FTM-8; shear strength 24 48 29 64 at roomtemperature, 10 N/cm² load, minutes to failure FTM-8; shearstrength >5000 >5000 >5000 >5000 at 100° C., 2 N/cm² load, minutes tofailure FTM-1; adhesive force 17.1 22.3 16.1 21.2 (180° peel resistance)N/in. Type of failure 25% c 100% c 100% c (cohesive = c; 75% a 100% aadhesive = a)

TDI-H contains aromatic isocyanate groups, it has a low equivalentweight compared with TDI-U and has different dissolution and reactionbehaviour.

The Examples show that certain properties already exhibit a maximum ondrying at 60° C. In the case of drying at 60° C., the system already haspredominantly cohesive properties, in the case of heat treatment at 100°C. it develops adhesion to the substrate: the adhesive properties resultin the cohesive properties being inferior to the adhesive properties inthe peel test according to FTM-1.

1. A process for preparing a pressure-sensitive adhesive layer on asubstrate comprising: i) applying an aqueous dispersion or solutioncontaining: a) at least one isocyanate-reactive polymer; and b) at leastone solid surface-deactivated polyisocyanate to at least one surface ofsaid substrate; ii) drying the aqueous dispersion or solution at atemperature of from 20 to 95° C. to form an adhesive layer that exhibitsa loop tack of at least 2 N/inch; iii) treating said adhesive layer withheat at a temperature that exceed the drying temperature by at least 40°C. to thereby produce a pressure-sensitive adhesive layer having a shearstrength higher than the shear strength of said adhesive layer.
 2. Theprocess of claim 1 wherein the drying temperature is from 50 to 95° C.3. The process of claim 1 wherein the treatment with heat is carried outat a temperature from 60 to 200° C.
 4. The process of claim 1 whereinthe treatment with heat is carried out with direct heat, convectionheat, radiation heat, electromagnetic radiation or by heat transfer froma preheated substrate.
 5. The process of claim 1 wherein theisocyanate-reactive polymer has a glass transition temperature less thanor equal to −5° C.
 6. The process of claim 1 wherein the aqueousdispersion or solution has a minimum film-forming temperature less thanor equal to +5° C.
 7. The process of claim 1 wherein theisocyanate-reactive polymer has a storage modulus less than or equal to10⁷ Pa, measured at 10 Hz and +10° C.
 8. The process of claim 1 whereinthe solid surface-deactivated polyisocyanate has a mean particlediameter (weight average) less than or equal to 10 μm.
 9. The process ofclaim 1 wherein the isocyanate-reactive polymer is an acrylic ester,methacrylic ester, acrylonitrile, vinyl acetate, styrene, ethylene, abutadiene or α-chlorobutadiene copolymer, a polyester- orpolyether-based polyurethane or a polyester- or polyether-basedpolyurea.
 10. The process of claim 1 wherein the ratio of isocyanategroups of the polyisocyanate to isocyanate-reactive groups of thepolymer is in the range of from 0.1 to 1.5.
 11. The process of claim 1wherein said pressure sensitive layer has a crosslinking density of atleast 3000 g/mol.
 12. The process of claim 1 wherein saidpressure-sensitive adhesive layer has a surface tackiness that isreduced by no more than 50% of the surface tackiness of said adhesivelayer.
 13. The process of claim 1, wherein the shear strength of saidpressure-sensitive layer exceed the shear strength of said adhesivelayer by at least 10%.